1. Technical Field
The present invention relates to radio direction finding systems and, more specifically, to systems of the interferometer type.
2. Prior Art
Interferometer type DF systems use at least two spaced antennas which each receive a radio signal the bearing direction of which is to be found. Comparison of the phases of the signals received by the antennas allows a number of possible bearings for the radio source to be calculated. The multiple bearings calculated are referred to as ambiguities as only one bearing value is correct. DF systems of this type become more accurate as the spacing between the antennas is increased relative to the wavelength of the radio signal. This spacing is referred to as the "aperture". A narrow aperture system, i.e. one where the spacing between the antennas is less than one wavelength, will produce few ambiguities but may be subject to considerable error in the possible bearing values calculated, particularly in a case where a significant proportion of the signal energy is being received by the antennas after reflection from some intermediate object. A wide aperture system is more accurate but produces a large number of ambiguities. For example a pair of antennas defining a narrow aperture of width equal to about a quarter of a wavelength or less will produce only one ambiguity, whereas a one wavelength aperture will produce three ambiguities and a wide aperture equal to five times the wavelength will produce 23 ambiguities. However for a five wavelength aperture the error in the bearing will be considerably smaller than that produced by the narrow aperture even in a case where the received signal includes a relatively large reflected component.
A single aperture is not very sensitive for detecting the bearing of a received radio signal when the source is lined up with the antennas defining the aperture.
In order to provide 360.degree. coverage, that is good sensitivity to radio sources at any bearing it has been proposed to use an arrangement of three antennas in a triangular configuration, for example a right angled triangle or an equilateral triangle, in order to define two apertures transverse to one another. If the apertures between the antennas are chosen to be wide then this type of arrangement gives good sensitivity and accuracy but still produces an unacceptably large number of ambiguities. For example using an isosceles right angled triangle with the equal perpendicular apertures each approximately twice the wavelength of the radio source, ten possible bearings, some of which may be coincident, are calculated for each aperture with a typical 30 degree spacing.
It is also known that these ambiguities can be resolved at least to some extent by using such a wide aperture interferometer system together with a narrow aperture system. Such systems may each use separate antennas or have one antenna in common. Typically both systems will use at least three antennas. Such a system is described in GB-2101440A in the name of Racal-SES Limited. In such a combined system the approximate but unambiguous bearing output from the narrow aperture system is compared with the multiple outputs from the more accurate wide aperture system. The closest one of these multiple outputs to the unambiguous output is output as the bearing of the radio source. A problem encountered with such systems arises out of this comparison. For example if the wide aperture system has an aperture equal to five wavelengths the multiple bearing outputs will be closely spaced at typically 7.2 degrees. If the narrow aperture system has an error of greater than half this angular spacing it is possible for the wrong one of the multiple wide aperture bearings to be chosen as the output. This effectively increases the error.
The technical problem of providing an accurate direction finding system using a small number of antennas and receivers remains to be solved.
Other prior art systems have used very large circular arrays of antenna, where each antenna is spaced less than half a wavelength from its neighbour. Such systems are described in GB-No. 1536996A and GB-No. 1455929A. These systems require large numbers of antennas together with complex processing procedure in order to provide adequate wide aperture accuracy. It has hitherto not been thought possible to use such a large circular array in an interferometer type DF system with wide aperture spacings between adjacent antennas.
Therefore enabling a practical interferometer type DF system to be formed with a wide aperture large circular array presents a further technical problem.